1. Field of the Invention
The present invention relates to a multi-beam image forming apparatus.
2. Description of the Related Art
There has been conventionally widely known an electrophotographic image forming apparatus which includes a light source unit for outputting a light beam such as a laser beam and forms an electrostatic latent image on a surface of an image carrier such as a photoconductive drum by scanning the surface of the photoconductive drum with a light beam output from the light source unit while reflecting the light beam toward the surface of the photoconductive drum by reflecting surfaces of a rotary polygon mirror. In an image forming apparatus of this type, a multi-beam method has been proposed for the purpose of shortening a time required for image formation. The multi-beam method is a method for forming an electrostatic latent image by simultaneously irradiating a plurality of light beams in parallel to an image carrier such as a photoconductive member by a light source unit of a plurality of light sources respectively for outputting light beams.
In a multi-beam image forming apparatus, the phases of respective light beams may be shifted in a main scanning direction due to an error in mounting semiconductor lasers, maladjustment and the like. A technology combining the following four points has been proposed to overcome this problem. The first point is to form a first pattern group by repeatedly forming a first pattern in a main scanning direction and a sub scanning direction. The first pattern is formed by one and another image patterns. The one image pattern is the one formed by repeating an operation of forming a certain dot row extending in the main scanning direction on a photoconductive member by a light beam from one of four semiconductor lasers in the sub scanning direction by as many (four) cycles as light beams. The other image pattern is the one formed by repeating an operation of forming a certain dot row extending in the main scanning direction on the photoconductive member by a light beam from the next semiconductor laser in the sub scanning direction by as many (four) cycles as light beams. The second point is to form a second pattern group by repeatedly forming a second pattern in the main scanning direction and the sub scanning direction. The second pattern is a pattern obtained by mirroring the first pattern in the main scanning direction. The third point is to set a plurality of combinations each comprised of two semiconductor lasers, which output adjacent light beams, out of four semiconductor lasers and form the first and second pattern groups for each combination. The fourth point is to detect the presence or absence of a phase shift in the main scanning direction between one and the other light beams in each combination according to whether or not the print density of the first pattern group and that of the second pattern group differ in each combination.
However, with the above technology, in the case of increasing the number of light sources installed in the light source unit without considerably increasing the size of the light source unit, a distance between arrival positions of two beams in each combination becomes narrower as the number of the installed light sources increases. Thus, differences between the width and area of a region where toner is attached in the first pattern group and those of a region where toner is attached in the second pattern group become smaller. In other words, as the number of the installed light sources increases, a difference between the print density of the first pattern group and that of the second pattern group becomes smaller, wherefore accuracy in detecting the presence or absence of the phase shift may be possibly reduced. Hence, the above technology is thought to be improper for the detection of the presence or absence of a phase shift in a light source unit including many light sources.